Audio coding systems use different methodologies for coding of audio, such as pure waveform coding, parametric spatial coding, and high frequency reconstruction algorithms including the Spectral Band Replication (SBR) algorithm. The MPEG-4 standard combines waveform coding and SBR of audio signals. More precisely, an encoder may waveform code an audio signal for spectral bands up to a cross-over frequency and encode the spectral bands above the cross-over frequency using SBR encoding. The waveform-coded part of the audio signal is then transmitted to a decoder together with SBR parameters determined during the SBR encoding. Based on the waveform-coded part of the audio signal and the SBR parameters, the decoder then reconstructs the audio signal in the spectral bands above the cross-over frequency as discussed in the review paper Brinker et al., An overview of the Coding Standard MPEG-4 Audio Amendments 1 and 2: HE-AAC, SSC, and HE-AAC v2, EURASIP Journal on Audio, Speech, and Music Processing, Volume 2009, Article ID 468971.
One problem with this approach is that strong tonal components, i.e. strong harmonic components, or any component in the high spectral bands that is not nicely reconstructed by the SBR algorithm will be missing in the output.
To this end, the SBR algorithm implements a missing harmonics detection procedure. Tonal components that will not be properly regenerated by the SBR high frequency reconstruction are identified at the encoder side. Information of the frequency location of these strong tonal components is transmitted to the decoder where the spectral contents in the spectral bands where the missing tonal components are located are replaced by sinusoids generated in the decoder.
An advantage of the missing harmonics detection provided for in the SBR algorithm is that it is a very low bitrate solution since, somewhat simplified, only the frequency location of the tonal component and its amplitude level needs to be transmitted to the decoder.
A drawback of the missing harmonics detection of the SBR algorithm is that it is a very rough model. Another drawback is that when the transmission rate is low, i.e. when the number of bits that may be transmitted per second is low, and as a consequence thereof the spectral bands are wide, a large frequency range will be replaced by a sinusoid.
Another drawback of the SBR algorithm is that it has a tendency to smear out transients occurring in the audio signal. Typically, there will be a pre-echo and a post-echo of the transient in the SBR reconstructed audio signal. There is thus room for improvements.
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